Neutron absorbers

ABSTRACT

The use of nitrogen as a neutron absorber is applied by employing silicon nitride in a foamed or high density form. Materials having neutron absorption cross-sections higher than silicon nitride may be incorporated with the silicon nitride.

Muted States Patent 11 1 1111 3,769,160 Shepherd et a1. Oct. 30, 1973 1NEUTRON ABSORBERS 3,344,080 9/1967 751 Inventors: Leslie RobertShepherd, Broadstone; Samuel Brim Hosegood, 3:261: 0o 7 19 Wareham, bothof England 3,142, 49 7 19 4 3 565 762 2/1971 [7 3] Assrgnee: UnitedKingdom Atomic Energy Authority, London England 3,516,948 6/1970 Cledatet a1. 252/478 Filed: y 1971 FOREIGN PATENTS OR APPLICATIONS 1,219,6961/1971 Great Britain 176/93 R [21] Appl. No.: 143,191

OTHER PUBLICATIONS Samsonov et 3.1., High Temperature Electric InsulatorPfmmy Dam and Ceramics, 1967, Nucl. Sci. Abstracts, Vol. 23,

May 21, 1970 Great B11131]! Dec. No- 52 US. (:1 176/86 R, 176/33,176/93, $32 33 gi z gr g rgf i? 2:2 ki

252/478, 423/290, 423/291, 423/344 g g [51] Int. Cl. G211 1/06,G2lf1/08, G210 11/00 58 Field of Search 252/478; 176/93, ""3"" Padgett176/86 R, 33; 23/110, 356, 3512-, 264/05; Em'mw-mger 9 23 29 291 34Attamey--Lars0n, Taylor & "111118 [56] References Cited [57] ABSTRACT UNE STATES PATENTS The use of nitrogen as a neutron absorber is applied2,988,522 6/1961 Smith et a1 252/478 by employing silicon nitride in afoamed or high den- 3,227,624 1/1966 Lechevallier 176/86 R sity form.Materials having neutron absorption cross- J 3 10/1963 Adamsky at252/478 X sections higher than silicon nitride may be incorpo- 3,326,8206/1967 Cuomo et al. 252/478 51]} fi h h "T i 3,356,618 12/1967 Rich etal 176/93 R 3,153,636 10/1964 Shanta et a1. 252/478 9 Claims, N0Drawings NEUTRON ABSORBERS BACKGROUND OF THE INVENTION This inventionrelates to neutron absorbing materials. Absorbers for absorbing thermalneutrons are em-- ployed in nuclear reactors for reactivity controlpurposes and for shielding parts from neutron irradiation. Suitableelements for absorption are easily selectable from inspection of a listof relevant cross sections and problems only arise in adapting theselected absorber in a form suited to in-core use, or in connection withthe physical, chemical or radioactive nature of the products of neutroncapture in the selected material.

SUMMARY OF THE INVENTION The present invention provides a means ofadapting the element nitrogen as a thermal absorber in the environmentof a high temperature gas cooled reactor.

According to the invention, a thermal neutron absorber for a hightemperature nuclear reactor comprises a solid ceramic body at leastincorporating silicon nitride. The ceramic body may contain nitrogen inits stoichiometric proportion or may be contained in an over-pressure ofnitrogen as by being enveloped in a gas tight container. In both forms,the ceramic body may be used as a control rod for a nuclear reactor orthermal "m ssa e's.-

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DESCRIPTION OF THE PREFERRED EMBODIMENTS pieces of absorbing ceramicmaterial would not fall out of the reactor core.

It is a known property of stoichiometric silicon nitride thatdenitridation tends to occur at very high temperatures and in anatmosphere substantially devoid of nitrogen gas, such as helium reactorcoolant gas. Obviously steps would have to be taken to limit thetempera- 5 gen to the gas coolant of the reactor.

When employed as thermal shielding permanently located in the reactorthe silicon nitride, which may be foamed to provide thermal insulation,must either be sufiiciently thick to provide adequate thermal neutronabsorption due to its nitrogen content, or, if requiring the addition ofboron, must contain sufficient boron to last the reactor life time.

Methods by which the article of silicon nitride neutron absorber may bemade are as follows:

Example I Powdered silicon was placed in a rubber mould and subjected toisostatic pressing to produce a coherent compact. The compact is thenitrided up to 10% by firing in nitrogen. The compact is then machinedto the 0 required shape. The machined compact is then fully nigeous forit can be made to assimilate sufficient boron 40 powder, for example, torender it a very strong absorber, a black absorber, in fact, and enableit to be used as a shutdown control rod. For example, boron ni-' trideor boron carbide may be mixed with silicon powder in proportions between30% to 1% or less boron compound to 70% to 99% or more silicon nitride,and the mixture pressed isostatically in a manner known per se. Apreferred proportion is 20% boron compound to 80% silicon nitride. Underpressure, the mixture is partially nitrided by firing in nitrogen and,at this stage, it may be machined, or otherwise shaped, also in knownmanner. After shaping, the compact is fully nitrided by firing at aboutl,400 C in nitrogen for a second, longer, period of time.

However, other known techniques of making bulk silicon nitride may beapplied to the powder mixtures as starting material.

Where concern is felt about brittleness and poor resistance tomechanical shock, of control rods made from these materials, one or moreof the following possibilities may be tried. One may incorporate in thematerial filaments of refractory metals, such as molybedenum ortungsten, one may enclose the control rods in filamentary stockings ofsuitable refractory material or one may enclose the control rods in cansof high temperature materials so that in the event of breakage, the

trided and has a density of 2.5 Example 11 As Example I except for thefact that the powdered silicon is mixed initially with boron containingpowder. Example III Silicon nitride powder is hot pressed at l,700 Cwith the addition of a suitable binder.

Example IV As Example III but boron nitride is mixed-with the siliconnitride starting material in weight proportions 20% boron nitridesilicon nitride.

Example V A silicon nitride article can be produced by flamesprayingsilicon powder at about 1,400 C on to a surface which has beenpreviously coated with a release agent, e.g., common salt, under normalatmospheric conditions. At 1,400 C the grains, being partially molten,bind together and partial nitridation occurs due to reaction withatmospheric nitrogen. When a sufficient thickness has been built up, theresulting partially nitrided silicon layer is removed and after finalmachining (if desired) full nitridation is carried out in the nor-. malmanner.

Any other process can be employed,which relies on light pressing atmoderate temperatures of the silicon bitride powder, to produce a foamedmaterial, there being mixed with the powder granules of a volatilenature, such as sugar balls, in any desired proportion. After lightpressing the material is nitrided during which the solid granulesvolatilise and leave voids so that the final product is foamed siliconnitride material. Such material is useful where properties Of neutronabsorption and heat insulation are required in combination. Here againthe neutron absorption properties may be enhanced by incorporating aboron compound in the mix.

We claim:

1. In a high temperature nuclear reactor, a thermal neutron shielding orcontrol rod element comprising a solid ceramic body consistingessentially of from 70 to 100 percent silicon nitride and from O to 30percent by weight of a boron-containing material.

2. The high temperature nuclear reactor according to claim 1 whereinsaid element comprises a control rod.

3. The high temperature nuclear reactor according to claim 2 whereinsaid ceramic body is enclosed in a nitrogen atmosphere within a gastight container.

4. The high temperature nuclear reactor according to claim 3 whereinsaid ceramic body is porous.

5. The high temperature nuclear reactor according to claim 3 whereinsaid neutron absorber material comprises a nitride or carbide of boron.

2. The high temperature nuclear reactor according to claim 1 whereinsaid element comprises a control rod.
 3. The high temperature nuclearreactor according to claim 2 wherein said ceramic body is enclosed in anitrogen atmosphere within a gas tight container.
 4. The hightemperature nuclear reactor according to claim 3 wherein said ceramicbody is porous.
 5. The high temperature nuclear reactor according toclaim 3 wherein said neutron absorber material comprises a nitride orcarbide of boron.
 6. The high temperature nuclear reactor according toclaim 1 wherein said solid ceramic body includes a boron-containingmaterial in an amount of from 1 to 30 percent by weight.
 7. The hightemperature nuclear reactor according to claim 1 wherein said ceramicbody includes reinforcing filaments of a refractory metal.
 8. The hightemperature nuclear reactor according to claim 6 wherein said refractorymetal is selected from the group consisting of molybdenum and tungsten.9. The high temperature nuclear reactor according to claim 1 whereinsaid neutron absorber element comprises shielding.